Hello Lauriane Lidove-Majcherek,
I understand that you are experiencing an issue with your Paint 3D projects disappearing. I'm sorry to hear that. As a Microsoft customer support agent, I can suggest a few troubleshooting steps that may help you recover your missing projects.
Firstly, please check if the projects are still saved in the default location. The default location for Paint 3D projects is "Documents\Paint 3D\Projects". If you have saved your projects in a different location, please check there as well.
If you are unable to find your projects in the default location or any other location, it is possible that they may have been accidentally deleted or lost due to a technical issue. In this case, I would recommend checking your Recycle Bin to see if the projects are still there. If they are, you can restore them from the Recycle Bin.
If you are still unable to locate your projects, you may want to consider using a data recovery tool to try and recover the files. There are several third-party data recovery tools available online that you can use to recover lost or deleted files.
I hope this information helps you recover your missing Paint 3D projects. If you have any further questions or concerns, please feel free to ask.
Best regards,
Bblythe Xiao